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[RFC] Improve core types for (Async) Iterable/Iterator/Generator #3990

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[RFC] Improve core types for (Async) Iterable/Iterator/Generator #3990

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40 changes: 23 additions & 17 deletions lib/core.js
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Original file line number Diff line number Diff line change
Expand Up @@ -61,8 +61,9 @@ declare class Object {

// Well known Symbols.
declare class $SymbolHasInstance mixins Symbol {}
declare class $SymboIsConcatSpreadable mixins Symbol {}
declare class $SymbolIsConcatSpreadable mixins Symbol {}
declare class $SymbolIterator mixins Symbol {}
declare class $SymbolAsyncIterator mixins Symbol {}
declare class $SymbolMatch mixins Symbol {}
declare class $SymbolReplace mixins Symbol {}
declare class $SymbolSearch mixins Symbol {}
Expand All @@ -76,8 +77,9 @@ declare class Symbol {
static (value?:any): Symbol;
static for(key: string): Symbol;
static hasInstance: $SymbolHasInstance;
static isConcatSpreadable: $SymboIsConcatSpreadable;
static isConcatSpreadable: $SymbolIsConcatSpreadable;
static iterator: string; // polyfill '@@iterator'
static asyncIterator: $SymbolAsyncIterator;
static keyFor(sym: Symbol): ?string;
static length: 0;
static match: $SymbolMatch;
Expand Down Expand Up @@ -547,20 +549,22 @@ type IteratorResult<Yield,Return> =
| { done: true, value?: Return }
| { done: false, value: Yield };

interface $Iterator<+Yield,+Return,-Next> {
@@iterator(): $Iterator<Yield,Return,Next>;
next(value?: Next): IteratorResult<Yield,Return>;
}
type Iterator<+T> = $Iterator<T,void,void>;

interface $Iterable<+Yield,+Return,-Next> {
@@iterator(): $Iterator<Yield,Return,Next>;
}
type Iterable<+T> = $Iterable<T,void,void>;

interface Generator<+Yield,+Return,-Next> {
interface $Iterator<+Yield,+Return,-Next> extends $Iterable<Yield,Return,Next> {
@@iterator(): $Iterator<Yield,Return,Next>;
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I wish Flow didn't require an @@iterator method on Iterators, even though they typically have one that returns this by convention. (But I'm not sure how to use this without hacks in any case: #1163.)

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#6075 has some relevant discussion

next(value?: Next): IteratorResult<Yield,Return>;
+return?: <R>(value: R) => IteratorResult<Yield,R|Return>;
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A return definition would be great. :)

I'd think it should just be () => IteratorResult<Yield, Return> (same for $AsyncIterator below) as long as that doesn't break passing a generator where an iterator is expected.

If you're hand-writing a return method to handle abrupt completions, you normally wouldn't accept any value because none are passed on IteratorClose. See http://www.ecma-international.org/ecma-262/7.0/#sec-iteratorclose

+throw?: (error?: any) => IteratorResult<Yield,Return>;
}
type Iterator<+T> = $Iterator<T,void,void>;

interface Generator<+Yield,+Return,-Next> extends $Iterator<Yield,Return,Next> {
@@iterator(): Generator<Yield,Return,Next>;
next(value?: Next): IteratorResult<Yield,Return>;
return<R>(value: R): IteratorResult<Yield,R|Return>;
throw(error?: any): IteratorResult<Yield,Return>;
}
Expand All @@ -569,25 +573,27 @@ declare function $iterate<T>(p: Iterable<T>): T;

/* Async Iterable/Iterator/Generator */

interface $AsyncIterator<+Yield,+Return,-Next> {
@@asyncIterator(): $AsyncIterator<Yield,Return,Next>;
next(value?: Next): Promise<IteratorResult<Yield,Return>>;
}
type AsyncIterator<+T> = $AsyncIterator<T,void,void>;

interface $AsyncIterable<+Yield,+Return,-Next> {
@@asyncIterator(): $AsyncIterator<Yield,Return,Next>;
}
type AsyncIterable<+T> = $AsyncIterable<T,void,void>;

interface AsyncGenerator<+Yield,+Return,-Next> {
interface $AsyncIterator<+Yield,+Return,-Next> extends $AsyncIterable<Yield,Return,Next> {
@@asyncIterator(): $AsyncIterator<Yield,Return,Next>;
next(value?: Next): Promise<IteratorResult<Yield,Return>>;
+return?: <R>(value: R) => Promise<IteratorResult<Yield,R|Return>>;
+throw?: (error?: any) => Promise<IteratorResult<Yield,Return>>;
}
type AsyncIterator<+T> = $AsyncIterator<T,void,void>;

interface AsyncGenerator<+Yield,+Return,-Next> extends $AsyncIterator<Yield,Return,Next> {
@@asyncIterator(): AsyncGenerator<Yield,Return,Next>;
next(value?: Next): Promise<IteratorResult<Yield,Return>>;
return<R>(value: R): Promise<IteratorResult<Yield,R|Return>>;
throw(error?: any): Promise<IteratorResult<Yield,Return>>;
}

declare function $asyncIterator<T>(p: AsyncIterable<T>): T;
declare function $asyncIterate<T>(p: AsyncIterable<T>): T;

/* Maps and Sets */

Expand Down
4 changes: 2 additions & 2 deletions newtests/array_literal_tuple_spread/test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -261,8 +261,8 @@ export default suite(({addFile, addFiles, addCode}) => [
test.js:3
3: const arr: Array<number> = [..."hello"];
^^^^^^ number. This type is incompatible with
276: @@iterator(): Iterator<string>;
^^^^^^ string. See lib: [LIB] core.js:276
278: @@iterator(): Iterator<string>;
^^^^^^ string. See lib: [LIB] core.js:278
`,
)
.because('String is an Iterable<string>'),
Expand Down
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